Structure and configuration of the passively q-switched diode end-pumped solid-state laser

ABSTRACT

The passively q-switched diode end-pumped solid-state laser is used the gain medium made of Er:Yb doped crystal and the Q-switch made of Co2+:MgAl2O4 crystal. The optical elements are optimally designed for the resonator to achieve pulse energy in a range 0.5 mJ&lt;E&lt;2mJ with the pulse width in a range of 4 ns-15 ns. The resonator is appropriate to use in laser rangefinders, target designator, and other products in military and civilian applications.

TECHNICAL FIELD

The present invention relates to a structure and configuration of the passively Q-switched diode end-pumped solid-state laser. This invention proposes a configuration that uses the end-pumping structure, can be applied for various military applications such as laser range-finder and target designator.

BACKGROUND

The structure of the passively q-switched diode end-pumped solid-state laser includes main components such as: laser diodes, coupling lenses, input coupler, active gain medium, q-switch and output coupler. These optical components are integrated by a precise mechanical housing to form a laser transmitter. The nanosecond pulse width is generated by using the active or passive q-switch. The pulse energy significantly depends on different parameters such as cavity length, active rod size, initial transmittance of saturable absorber and output coupler reflectivity. For the resonator cavity using an end-pump structure, it is important to optimize the pump laser beam diameter or dimensions to maximize the output peak power. In addition, the initial transmittance of the Q-switch and the reflectivity of the output coupler is also necessary to study improving the pulse energy and reduce the intracavity fluence, simultaneously. The present invention is focused on the configuration of a resonator cavity in which the mechanical, optical, and physical parameters are optimized to ensure that the cavity can achieve pulse energy in the range of 0.5 mJ to 2 mJ with the pulse width in the range of 4 ns to 15 ns.

SUMMARY

The purpose of the present invention is to propose a laser cavity configuration using an Er:Yb doped glass and Co₂₊:MgAl₂O₄ crystal as a passive q-switch. The technical specifications of elements are optimized to achieve pulse energy (E) in the range of 0.5 mJ to 2 mJ and laser pulse width in the range of 4 ns to 15 ns.

To achieve the above purpose, the proposed structure includes the following components:

A laser base is used to mount optical elements comprising of laser diode, coupling lenses, input coupler, gain medium, Q-switch, and output coupler along the optical axis of the diode laser to form a laser transmitter.

A diode laser source is a pulsed laser source that is a type of laser diode bar with a center wavelength in the range of 900 nm to 1000 nm, which can generate laser pulses with a peak power of 20 W to 120 W, the pulse width of 2 ms to 5 ms, and the pulse repetition rate of 1 Hz to 10 Hz.

Coupling lenses are responsible for guiding the laser from the diode laser source to the active gain medium in the cavity. The position of each lens in the system can be varied axially to control the diameter/dimensions of the pump laser beam entering the active rod in the range of 0.5 mm to 1 mm.

An input coupler that is coated with an anti-reflective layer for the pump laser and highly reflective layer for the emitted radiation. The input coupler can be an individual mirror or coated directly on the end-side of the gain medium.

A intracavity holder is used to attach the gain medium and the Q-switch. This intracavity holder is precisely mounted on the laser base.

A gain medium is made of phosphate glass doped with ions Er³⁺ and Yb³⁺ (Er: Yb co-doped crystal). The laser with the desired wavelength will be emitted under the excitation of the pump diode laser source.

A passive Q-switch is made of Co^(2+: MgAl) ₂O₄ material as a laser on/off switch can be placed perpendicularly or at Brewster's angle to the laser in the cavity.

An output coupler is coated to allow the laser to transmit partially. A part of the laser is reflected into the resonator cavity while the remaining part of the laser exits from the resonator cavity with the desired pulse energy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing the general structure of the passively q-switched diode-pumped solid-state laser.

FIG. 2 is a graph showing the absorption coefficient and the emission cross-section according to wavelength.

FIG. 3 is a graph showing the dependence of gain coefficient versus the rod length at different durations of the pump pulses.

FIG. 4 is a diagram showing the laser beam from the diode laser through the coupling lenses to the gain medium.

FIG. 5 is a drawing showing the simulation of the cross-section of the laser beam pumped in the gain medium.

DETAILED DESCRIPTION OF THE DISCLOSURE

FIG. 1 shows the configuration of a laser transmitter which mainly includes the laser base (1), laser diode source (2), coupling lenses (3) guiding diode laser into resonator cavity, input coupler (4), intracavity holder (5) to mount the gain medium (6) and passive Q-switch, and the output coupler (7).

The laser base (1) made of copper (Cu) is precisely machined, the mounting position between the input coupler and output coupler can be varied in a range of 15 mm-30 mm to ensure the output laser pulse width is in a range of 4 ns-15 ns.

The diode laser source (2) is mounted on the laser base (1). The center wavelength of the diode laser is in a range of 900 nm-1000 nm to match the absorption spectrum diagram of the gain medium made of Er: Yb doped glass as shown in FIG. 2. The diode laser source is a pulsed laser source that possesses pulse peak power in a range of 20 W-120 W, pulse width in a range of 2 ms-5 ms, and pulse repetition frequency in a range of 1 Hz-10 Hz. The pulse peak power in the mentioned band ensures adequate pump power for the resonator cavity to generate the laser pulse energy in a range of 0.5 mJ to 2 mJ. The pump pulse width in a range of 2 ms-5 ms is based on the gain medium material Er: Yb. The pulse emission frequency from 1 Hz to 10 Hz corresponds to the desired pulse repetition frequency of the output laser.

The coupling lenses consists of spherical and cylindrical lenses (FIGS. 3 and 4), which can be adjusted along the optical axis, to ensure that the diameter (D_(B)) or dimensions (a, b) of the cross-section of the pump laser beam in the gain medium are in a range of 0.5 mm-1 mm. L₁ and L₂ are two cylindrical lenses used to reduce the beam divergence of the pump beam horizontally and vertically, respectively (FIG. 4).

Gain medium (6) is made of Er: Yb doped glass which is doped from 0.3×10²⁰ to 0.5×10²⁰ cm⁻³ Er³⁺ ions and from 1.7×10²¹ to 2×10²¹ cm⁻³ Yb³⁺ ions. Yb³⁺ ion concentration in the above range are used to optimize the energy transfer efficiency from Yb³⁺ ions to Er³⁺ ions during pumping. Er³⁺ ion concentration from 0.3×10²⁰ to 0.5×10²⁰ cm⁻³ is used to obtain the maximum value of gain in the cavity and reduce the reabsorbing photon emitted in cavity.

The gain medium length is optimally calculated for the resonator cavity operating in free-run mode:

$\begin{matrix} {\frac{\partial N_{Y\; b}}{\partial t} = {{W_{p}\left( {x,y,z,t} \right)} - \frac{N_{Y\; b}}{\tau_{Y\; b}} - {\alpha_{ET}{N_{Yb}\left( {N_{Er}^{0} - N_{Er}} \right)}}}} & (1) \\ {\frac{\Delta\; N_{Er}}{\Delta\; t} = {{\alpha_{ET}{N_{Yb}\left( {N_{ER}^{0} - N_{ER}} \right)}} - \frac{N_{Er}}{\tau_{Er}}}} & (2) \end{matrix}$

where N_(Yb) and N_(Er) respectively are population density of Yb³⁺ at energy level ²F_(5/2) and Er³⁺ at energy level ⁴I_(13/2); N⁰ _(Er) is doping concentration of Er³⁺ ions; τ_(Er) and τ_(Yb) are the lifetime of Er³⁺ and Yb³⁺ ions at energy level mention above, respectively; W_(p) (x, y, z, t) is the rate of laser pump source; α_(ET) is energy transfer coefficient between Yb³⁺ and Er³⁺. Gain g(x, y, z, t) in gain medium with the length of z can be expressed by:

g(x,y,z,t)=∫₀ ^(z) k _(g)(x,y,z′,t)dz′  (3)

k _(g=) N _(Er)δ_(SE) ^(L)−(N _(Er) ⁰−N _(Er))δ_(abs) ^(L)   (4)

where δ_(SE) ^(L) and δ_(abs) ^(L) are emission and absorption cross-section of Er³⁺ at laser wavelength ν_(L). FIG. 3 shows one round-trip gain as a function of gain medium length with different pump pulse-widths. Optimally, gain medium length ranges from 5 mm to 8 mm while pump pulse width ranges from 2 ms to 5 ms.

Input coupler (4) is coated with the anti-reflective layer for the wavelength between 900 nm and 1000 nm to transmit >98% and highly reflective for the wavelength between 1525 nm and 1570 nm to reflect >98%. This element can be an individual mirror or a coating layer on the end-side of the gain medium (6).

Passive Q-switch (7) is a saturable absorber made of Co⁺:MgAl₂O₄ crystal with initial transmittance (T₀) in the range 85%<T₀<92%, which ensures laser pulse energy >0.5 mJ.

Intracavity holder (5) attaches gain medium (6) and Q-switch (7). Q-switch (7) is placed perpendicularly or at Brewster's angle to the optical line. This element can be placed separately or bonded directly on the right side of the gain medium (6). The optical line is defined as the line perpendicular to the input (4) and the output coupler (8) which simultaneously is the symmetry axis of the gain medium (6). The intracavity holder (5) is precisely mounted on the laser base (1).

Output coupler (8) is coated to allow the radiation to reflect <85% with the wavelength between 1525 nm and 1570 nm, hence, the intracavity fluence in the cavity is <10 J/cm² (damage threshold of the optical elements used). The intracavity fluence (F_(in)) is expressed by:

$\begin{matrix} {F_{in} = {{\frac{2 - T_{OC}}{T_{OC}}\frac{2E}{S_{SA}}} < {10\mspace{14mu}{J/{cm}^{2}}}}} & (5) \end{matrix}$

where T_(OC) is the transmittance of output coupler, E is pulse energy output from the laser system.

Although the structure of the resonator cavity in the present invention has been illustrated in the accompanying Drawings and described in the foregoing Detailed Description, it should be noted that the invention is not limited to the described resonator cavity, but is capable of different rearrangements, modifications or substitutions without departing from the invention as set forth and defined by the following claims. 

what is claimed is:
 1. A configuration of a passively q-switched diode end-pumped solid-state laser, comprising: a laser base attaching optical elements along an optical axis of a diode laser source to form a laser transmitter; The diode laser source is a pulsed laser source with a center wavelength in a range of 900 nm-1000 nm, which can generate pulsed laser with a peak power in a range of 20 W-120 W, a pulse width in a range of 2 ms-5 ms, and a pulse repetition frequency in a range of 1 Hz-10 Hz; coupling lenses which are responsible for guiding the pulsed laser from the diode laser source into a gain medium in the resonator cavity, a position of the coupling lenses varied axially to control a diameter of a pump laser beam entering the gain medium in a range of 0 5 mm-1 mm; an input coupler which is anti-reflective coating for the diode laser beam and highly reflective coating for the radiations, which possess a desired laser wavelength, emitted in the resonator cavity, wherein the input coupler comprises an individual coupler or a coating directly on an end-side of the gain medium; an intracavity holder attaching the gain medium and a Q-switch, wherein the intracavity holder is precisely mounted on the laser base, the Q-switch is placed perpendicularly or at a Brewster's angle to an optical line, wherein this element is placed separately or coated directly on a right side of the gain medium, the optical line is defined as a line perpendicular to the input coupler and output coupler which simultaneously is a symmetry axis of the gain medium; the gain medium is made of glass doped with ions Er³⁺ and Yb³⁺, wherein the laser with the desired wavelength will be emitted under the excitation of a pump diode laser source, the passive Q-switch is made of Co⁺:MgAl₂O₄ crystal as a laser on/off switch and is placed perpendicularly or at Brewster's angle to the laser in the resonator cavity, the output coupler is coated to allow the laser to transmit partially, a part of the laser is reflected into the resonator cavity while a remaining part of the laser exits from the resonator cavity with a desired pulse energy.
 2. The configuration of claim 1, wherein the resonator cavity uses a gain medium made of Er:Yb doped glass with an Er concentration in a range of 0.3×10²⁰ to 0.5×10²⁰ cm⁻³, Yb concentration in a range of 1.7×10²¹ to 2×10²¹ cm⁻³, gain medium radius 0.5 mm<R_(Er:Yb)<1, mm and gain medium length 5 mm<L_(Er:Yb)<8 mm.
 3. The configuration of claim 1, wherein the end-pumped laser transmitter has a mechanical length between two couplers (input and output couplers) 15 mm<L_(KÐ)<30 mm.
 4. The configuration of claim 1, wherein the Q-switch is made of Co⁺:MgAl₂O₄ crystal with initial transmittance 85%<T₀<92% and reflectivity of output coupler <85% at wavelength 1525 nm-1570 nm.
 5. The configuration of claim 1, with coupling lenses that can adjust positions of individual lens to change a diameter or cross-section dimensions of pump beam from laser diode source in the range: 0.5 mm<D_(B), a, b<1 mm 